Holographic optical element

ABSTRACT

A holographic optical element comprising at least two sets of regions, the regions of each set being different from the regions of the other set(s) and being interleaved or overlapping with the regions of the other set(s) and being constructed such that light incident on each set of regions is directed to a respective one of a plurality of viewing zones. 
     The holographic optical element is typically incorporated in a display device such as a stereoscopic display device.

CROSS REFERENCE TO RELATED APPLICATION

This Application is a divisional of the U.S. patent application Ser. No.08/750,364, filed Dec. 5, 1996, now U.S. Pat. No. 6,157,474.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a display device, and moreparticularly to a display incorporating a holographic optical element.

2. Description of Background Art

In our earlier application WO93/02372 we describe a display deviceincorporating a holographic optical element. Light incident on theholographic optical element is directed to a single viewing zone. Bymoving the source of the light, the position of the viewing zone can bemoved. Temporally alternating left and right images are projected on thescreen from alternating positions whereby the images are viewable oneafter the other in respective left and right hand viewing zones.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, we provide adisplay device comprising a holographic optical element having at leasttwo sets of hologram regions, the region(s) of the first set beinginterleaved or overlapping with adjacent region(s) of the second set(s)and being constructed such that light incident on each set of regions isdiffracted so as to construct a respective one or more of a plurality ofreal or virtual images of a diffuse light source; and image generatingmeans comprising a plurality of image elements; wherein the holographicelement and its hologram regions are disposed and designed such thatlight diffracted by the first set of hologram regions passes through acorresponding first set of image elements and such that light diffractedby the second set of hologram regions passes through a correspondingsecond set of image elements.

According to a second aspect of the present invention, we provide adisplay device comprising a holographic optical element having at leasttwo sets of hologram regions, the region(s) of the first set beinginterleaved or overlapping with adjacent region(s) of the second set(s)and being constructed such that light incident on each set of regions isdiffracted so as to construct a respective one or more of a plurality ofreal or virtual images of a diffuse light source; and image generatingmeans comprising a plurality of image elements; wherein the holographicoptical element and its hologram regions are disposed and designed suchthat light having passed through a first set of image elements of theimage generating means is diffracted by the first set of hologramregions and such that light having passed through a second set of imageelements of the image generating means is diffracted by the second setof hologram regions.

Each set may comprise a single region, which extends across a largeproportion of the holographic optical element. Typically however, eachset of regions comprises an array of regions.

Typically the regions making up an array are laterally offset from eachother (e.g. vertically and/or horizontally).

Each region in an array is typically spaced from and does not overlapwith adjacent regions in the array, although adjacent regions in thesame array may overlap.

Also, each set of regions is typically laterally offset from the otherset(s) of regions. The regions within an array may overlap with regionsof other arrays. Typically however, the regions of each array do notoverlap with regions of other arrays.

The at least two sets of regions may make up a lateral array of verticalor horizontal stripes, or a two-dimensional array such as a honeycomb.

In a preferable embodiment, the holographic optical element comprisestwo interleaved sets of regions.

Typically the holographic optical element is constructed such that eachset of regions comprises a holographic recording of one or more diffuselight sources.

By making a recording of the same light source, or a number of differentlight sources in a plurality of different positions with respect to theholographic optical element, the light incident on the holographicoptical element is directed to a position corresponding with theposition(s) of the diffuse light source(s). Put another way, when eachof the sets of regions is illuminated with light, the diffracted lightforms an image of the one or more diffuse light sources. Typically theimage is a real image although it may also be a virtual image.

Where each set of regions comprises a recording of only one diffusesource, then light incident on the set of regions will be directed to asingle viewing zone. Where each set of regions comprises a recording ofa plurality of diffuse sources, then light incident on the set ofregions will be directed to a plurality of viewing zones.

The holographic optical element is typically a transmissive element,although it may be reflective.

The holographic optical element may be formed in any suitable way, forinstance as a surface relief hologram made e.g. by moulding orembossing, or as a volume hologram made in e.g. photopolymer,dichromated gelatine or silver halide.

Typically the holographic optical element is provided as part of adisplay device such as a stereoscopic display device. This provides aparticularly compact and easily constructed spatially multiplexed two orthree-dimensional display device with holographically generated viewingapertures and capable of the display of images in real time.

Typically the display device is provided with image generating meansdefining a plurality of images each corresponding to a respective arrayof image elements (such as an array of LCD pixels). Typically theholographic optical element is also recorded with a holographicrecording of an array of apertures. Where the element is incorporated ina display device, the array of apertures is arranged to coincide witharrays of alternating image elements in the display. By arranging theholographic optical element and the array of image elements in this way,light passing through the image elements is diffracted to a respectiveviewing zone by one of the sets of regions in the holographic opticalelement.

In a first example, the holographic element is placed adjacent to and infront (i.e. the viewer's side) of the LCD.

The LCD may be backlit from a single source. Alternatively the LCD maybe backlit from a number of sources grouped so as to be functionallyequivalent to a single source.

The source or sources may be moved in order to move the positions ofviewing zones. Alternatively the source may be static and its opticalimage moves (e.g. through the use of moving mirrors or a variety ofother means). In a further alternative an array of static sources or asingle elongated source with a controllable masking element may beprovided so that the effective position of the source is moved byswitching lights on/off or moving the masking element respectively.

Typically the LCD display carries a plurality of interleaved images (forinstance in the form of a set of lateral stripes or in a honeycombarray).

The plurality of images may comprise the same view of a two-dimensionalimage if a two-dimensional display is required. In this case the sourceis preferably a plurality of laterally offset sources or a linear source—so as to overlap the left and right viewing zones thereby providing afull resolution 2D image to both eyes.

Typically however, the plurality of images comprise left and right-handstereoscopic views. The regions of the holographic optical elementtypically correspond with the array of LCD image elements. Theholographic regions corresponding to the left-hand image elementsdiffract the light to (and hence render the image visible from) aleft-hand viewing zone and vice-versa for the right-hand image.

Alternatively the holographic optical element may be placed behind theLCD. In this case, the holographic optical element may be placedadjacent to the LCD, in which case the sets of regions will coincidewith the array of the LCDs. However, if the holographic optical elementis spaced from the LCD, then the regions of each set will typicallyoverlap with regions from the other set, or even with regions of theirown sets. In a limiting case, each set of regions may comprise a singleregion extending across the holographic optical element. However, theholographic optical element is still recorded and configured such thatlight, which passes through e.g. the left-hand LCD image elements, isonly directed to the left hand viewing zone, and vice-versa.

The holographic optical element may be constructed in a number of waysas described more fully in the following specification. However, commonfeatures of the recording method are that, at some stage in therecording, an image of a diffuse light source is recorded with the lightsource in a plurality of positions with respect to the holographicplate, each position relating to a respective viewing zone.

The holographic optical element may be recorded in a “one step” processin which a diffuse light source is recorded in a plurality of positionsin two exposures.

Typically however, the holographic optical element is recorded as atransfer hologram from one or more master hologram plates.

Another common feature of the holographic recording process is that atsome stage in the process, a mask (corresponding to the array of LCDimage elements in the display device) is used in the recording process.The mask may be placed adjacent to the holographic plate in therecording of either the master hologram or the transfer hologram.Alternatively, the mask may be placed a distance from the holographicplate in which case it can be considered that a holographic recording isbeing made of the viewing apertures created by the mask.

The holographic optical element may be constructed as a substrate guidedwave device.

In addition, the display device may further comprise a focusing optic,which creates a virtual image of the array of image elements and theholographic optical element and forms real images of the viewing zones.In such a case the image of the diffuse light source produced by theholographic optical element might advantageously be formed as a virtualimage in the first instance.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of embodiments of the invention will now be described withreference to the accompanying drawings, none of which are to scale, inwhich:

FIG. 1 illustrates a holographic optical element according to theinvention;

FIG. 2 is a perspective view of a holographic optical element accordingto the invention constructing a left hand and right hand viewing zone;

FIG. 3 is a schematic view of a first display device in which theholographic optical element is placed in front of the LCD;

FIG. 4 is a schematic view of a second display device in which theholographic optical element is placed behind the LCD and comprises anarray of overlapping regions;

FIG. 4A is a schematic view of a display device illustrating the effectof varying the separation between the hologram and the LCD;

FIG. 5 illustrates the recording of a master holographic plate for aholographic optical element according to the invention;

FIG. 6 illustrates the recording of the left hand viewing zone on themaster holographic plate;

FIG. 7 illustrates the recording of the right hand viewing zone on themaster holographic plate;

FIG. 7a illustrates an alternative diffuser arrangement for therecording of a viewing zone on the master holographic plate;

FIG. 8 illustrates the recording of a transfer hologram from the masterhologram recorded in FIGS. 6 and 7;

FIG. 9 illustrates the construction of a substrate guided wave transferhologram from the master hologram recorded in FIGS. 6 and 7;

FIG. 10 illustrates a second example of a substrate guided wave transferhologram;

FIG. 11 illustrates a display device incorporating the substrate guidedwave holographic element illustrated in FIG. 9 or FIG. 10;

FIG. 12 illustrates a further example of a holographic optical elementconstructed as a substrate guided wave device;

FIG. 13 illustrates a “one-step” method of fabricating the holographicoptical element; and,

FIG. 14 illustrates part of a display device incorporating an additionalfocusing optic; and

FIG. 15 illustrates the simultaneous formation of a number ofstereoscopic viewing zones.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments described allow the display of images in two or threedimensions. The display is autostereoscopic and can display full color,moving 3D images in real time. It is based on a two dimensional imagebearing medium such as a photographic transparency or an electronicdisplay such as a Liquid Crystal Display (LCD) used in conjunction witha novel holographic device and lighting system. In the followingdescription we will presume that the image bearing panel is a LCD.

Referring to FIG. 1, light source 1 emits light 2 which illuminates ahologram (i.e. holographic optical element) 3, which diffracts the lightin such a way as to reconstruct the wavefront 4 which forms an even anddiffusely illuminated real image 5.

The zone defined by the real image 5 may be considered to be a diffuselyilluminated plane at a distance from the hologram 3. In principle (andignoring the zero order which passes straight through the hologram) noneof the light 2 is diffracted outside of the zone 5.

When a point on the hologram 3 is viewed by an eye 6 it will appear tobe bright if a straight line drawn through it and the eye 6 passesthrough zone 5. Conversely it will appear dark if a straight line drawnthrough it and the eye 6 does not pass through zone 5.

The first case is illustrated by eye 6 a, which perceives all points onhologram 3 as being bright and the second by eye 6 b for which the wholesurface will be dark.

This principle applies both in the illustrated case where the zone 5 isa real image and in a case where the zone is a virtual image (i.e. whenit is an image which appears to form on the far side of the hologram 3,as viewed by the eye 6.)

The holographic techniques to be described below permit thereconstruction of image 5 in such a way that the light distribution iseven across its whole area and its borders can be precisely defined.This is advantageous in autostereoscopic applications as it assists inthe reduction of crosstalk between left and right views without theconverse problem of creating a dim or blank zone separating the left andright viewing zones.

Considering now the perspective drawing FIG. 2 (which is not to scale):light rays 2 shine on the spatially divided hologram 3 which consists ofa set of sections 3 l the members of which all reconstruct real image 5l; and a set of sections 3 r the members of which all reconstruct realimage 5 r. It will be noticed that the real images 5 are preciselylocated in space and they can be made so that they butt up to each otherwith neither a gap nor overlap between them.

Imagine the whole surface of the hologram 3 evenly illuminated by rays 2and consider the two exemplary rays 2 l and 2 r (both of which may beconsidered to originate at the same light source 1 (FIG. 1)). Ray 2 l isan exemplary member of the set of rays that strike the zones 3 l of thehologram 3. Some of the light 2 l is not diffracted and continues on inthe zero order light 7 (which is of little interest here). Thediffracted light (illustrated by the example ray 4 l) reconstructs thediffuse real image 5 l. Similarly the ray 2 r is diffracted by thehologram section 3 r reconstructing the real image 5 r.

An eye 6 l is positioned such that all straight lines drawn through theeye 6 l to any point on the hologram 3 will pass through the zone 5 l;eye 6 l will consequently see all the sections 3 l of the hologram 3light up, at the same time all the sections 3 r will remain dark for eye6 l as the same imaginary line does not pass though the zone 5 rreconstructed by the areas 3 r of the hologram 3. The converse appliesin the case of the eye 6 r, which will see the areas 3 r light up andthe areas 3 l will be dark.

If the eye 6 l is a viewer's left eye and 6 r is the same viewer's righteye then each eye sees a different part of the hologram 3 light up.Specifically, eye 6 l will perceive every member of the set of sections3 l to be bright and every member of the set 3 r to be dark. Theconverse applies for eye 6 r.

Up to this point we have presumed that the illumination of the hologram3 by light source 1 has been even and there has been nothing to vary theperceived intensity of the light diffracted towards the images 5. Thescreen is in effect blank. But the light impinging on the hologram 3 canbe made to pass through a LCD (liquid crystal display). If the displayis appropriately made there will be sections of the LCD lying onstraight lines drawn through the optical position of the light source 1and the sections 3 l of the hologram 3 and others on lines from source 1and sections 3 r.

If those sections of the LCD thereby corresponding to sections 3 l ofhologram 3 display a certain image L while those corresponding tosections 3 r display a second image R it will be clear that eye 6 l willsee image L while eye 6 r will see image R. If the two images L and Rare the components of a stereo pair then a stereoscopicthree-dimensional picture can be seen.

Similarly the light may be modulated by placing the LCD on the otherside of the hologram. In this case the light 2 is first diffracted bythe hologram and then passes through the LCD. In this case the hologramand light combination may be considered a sort of backlight. The light 4l forming image 6 l will pass through certain sections of the LCD whichmay be arranged to display image L, the light 4 r passes through othersections displaying image R. In this way the two eyes 6 l and 6 r againsee separate pictures, which may combine through stereopsis to show animage in three dimensions.

There are applications which will require the images 5 l and 5 r to formfirst as virtual images, in particular this may apply where a display ofthis sort is used with another focusing optic which will create avirtual image of the LCD and Hologram 3 combination and bring the images5 r and 5 l in front of the optic as real images. Such an arrangement isdescribed in greater detail below.

We can now describe the nature of a number of embodiments of theinvention in greater detail. (We will describe the case where there arejust a pair of images to be displayed, more can be displayed byappropriate changes. Similarly we will consider only one light source,though several may be used. We will consider just one stereo viewingposition while knowing that more can be provided and we will considerthe essential elements and ignore the fact that other optics —mirrors,lenses, holographic optical elements —may be included in order to assistin the practical construction and operation of the display.)

For the purposes of this discussion the composite hologram 3 may beconsidered to be composed of sets of ‘apertures’HL and HR such thatlight impinging (at an appropriate angle of incidence) on any member ofthe set HL reconstructs one image and light impinging on a member of theother set HR reconstructs a different image. These images form at adistance from the plane of the hologram 3 and may be virtual or real.According to the precise geometry these two sets of apertures HL and HRmay or may not overlap.

The two images 5 l and 5 r formed respectively by the two sets ofapertures HL and HR are evenly illuminated diffuse areas of light andmay themselves be considered as apertures IL and IR, where IL isreconstructed by set HL and IR by set HR.

Combined with the hologram is a LCD panel which will display twopictures A & B. The pixels of this LCD may also be considered asapertures and may be divided into two sets PL and PR; where the set PLdisplays picture A and the set PR picture B.

In addition a pair of apertures represent the viewer's two eyes. Theleft eye of the viewer is aperture EL and the right one is ER. Theapertures (or images thereof) IL and IR form at a distance from thehologram 3 and in the same general region as the eyes EL and ER. Optimumoperation of the display may be defined as the case where light rays canbe traced back from EL to the light source 1 such that they pass throughaperture IL, all members of the set of apertures PL and all members ofthe set of apertures HL, but not necessarily in that order. At the sametime no rays should be traceable from EL though IR, PR and HR to thesource 1. Similarly rays traced from ER must pass through IR, PR and HRand not through IL, PL and HL.

Two important configurations are illustrated in FIG. 3 and FIG. 4 whichare enlarged details of a section through the hologram 3 and the LCD 8.(The FIGs. are not to scale.)

FIG.3 represents a case where the hologram 3 is on the viewer's side ofthe LCD 8. It can be seen that a ray of light 2 l first passes through apixel which is a member of the set of pixels 9 l, it then strikes thehologram 3 in the position of a portion of the hologram 3 which is amember of the set of sections 3 l, it is then diffracted. The zero order7 passes straight through the 1st order wavefront is represented as raysof light 4 l and reconstructs the image of the zone 5 l. The sameapplies mutatis mutandis to rays 2 r, pixel set 9 r, hologram sectionset 3 r, diffracted rays 4 r and image 5 r. The hologram 3 and LCD 9should be mounted close to each other and such that all the rays passingthrough pixels set 9 l will reconstruct only image 5 l and similarly therays passing through pixel set 9 r should form only image 5 r. This endmay be assisted by introducing blank areas in the hologram 3 between themembers of the set 3 l and those of 3 r.

Moving to FIG. 4: in this case, also from the viewer's position, thehologram 3 is mounted behind the LCD 9. It should first be noted thatthe hologram described in relation to FIG. 3 can also work in thissituation provided the optical path length between the holographicsurface and the pixels of the LCD is kept small (in practice usually notgreater than about four times the pixel spacing). FIG. 4 does, however,represent an improved arrangement which allows for some increase inseparation of the hologram 3 and the pixels of the LCD 9.

Light 2 strikes the hologram 3. Where it strikes a section of thehologram, which is a member of the set 3 l it is diffracted in such away as to pass first through a member of the set of pixels 9 l and thenon to reconstruct the image 51. The same applies mutatis mutandis tolight striking an area of the hologram containing a member of the set 3r. It will be noticed in this case that the members of the set 3 loverlap spatially with members of the set 3 r. It is even conceivablethat they would also overlap with members of their own set. It shouldalso be noticed that the display can be configured so that every memberof the set of pixels 9 l receives light only from hologram elementsmembers of the set 3 l and that similarly every member of the set ofpixels 9 r receives light only from hologram elements members of the set3 r.

The overlapping of members of the same set is illustrated in FIG. 4A.FIG. 4A shows some light rays 4 l traced through the hologram and LCD.If the separation is relatively small (hologram position 1) thedifferent members of the same set 3 l will not overlap each other(though in the illustrated case they would overlap members of the otherset 3 r which are omitted from the drawing). If the separation isincreased (hologram position 2) we find that they overlap on each otheras the zones occupied by each member of the set widen.

We will now describe some of the practical characteristics of a displaymade according to this method.

We have described elsewhere (PCT/GB 93/01709) how the position of aviewing zone for a holographic autostereoscopic display may be moved bymoving the effective position of the light source. This principle may beapplied here in the following basic ways:

The viewing zone may be moved by moving the position of the light sourcethereby allowing the display to be controlled so that the viewer maymove without loss of stereo effect. If the stereo viewpoints of thedisplayed images are appropriately updated the impression of parallaxcan be provided allowing the viewer to look around and even above andbelow and closer and further with the parallax of the image changingaccordingly.

Separate sources may be provided thereby creating two or more separatestereo viewing zones so that more than one person can enjoy the stereoeffect at a time. In this case the effective position of each of theseparate sources may also be movable thereby allowing each viewermobility.

In the above the effective position of the source(s) can be changed soas to compensate for the viewer(s) changing positions side to side, upand down and in distance. In practice the side-to-side movement islikely to be the most important.

Separate sources or an extended source may be provided to overlap thezones 5 l and 5 r over each other thereby allowing both eyes to see allthe pixels of the LCD. This procedure is advantageous as it displays afull resolution 2D image to both eyes thereby allowing conventional twodimensional pictures to be used and enjoyed without any compromise totheir quality —this is particularly helpful for all conventional 2D usesof such a display. Clearly with this illumination the autostereoscopiccapability is temporarily disabled. Such capability is easily turned onand off either automatically or manually.

An additional conventional diffuse backlight may be provided, this willbe located behind the hologram 3 and behind the LCD (seen from theviewer position). Such an arrangement will provide an alternative methodof achieving 2D operation. This is most conveniently achieved if thehologram 3 is made as a Substrate Guided Wave (SGW) hologram.

We will now describe one method of making the hologram 3 and indicatesome of the many variations on the technique. We will concentrate on theessentials leaving out many parts relating to the optical geometriesthat may be used and which will be familiar to holographers.

We will first describe the making of a first generation hologram or H1.

Referring to FIG.5 (which is a schematic view and is not to scale) aholographic plate 10 is exposed simultaneously to a reference beam 11(preferably collimated) and to the object light 19 l. The requiredobject light 19 l is produced by illuminating a diffuser 12 (e.g. aground glass screen) with a beam of light 14; the extent of the diffusearea visible to the plate 10 is restricted by mask 13 l; light 14 isscattered by the diffuser 12 towards mask 16; some rays 18 l of thescattered light 15 impinge on the opaque areas 16 r of mask 16 and areblocked; other rays 19 l pass unimpeded through the clear areas 16 l ofmask 16 and then impinge on the holographic plate 10 where theyinterfere with the reference beam 11, the interference fringes therebyproduced are recorded in the normal way. This constitutes the firstexposure of master hologram 10.

A second exposure is then made with the mask 13 l replaced with mask 13r (which might be mask 13 l translated by, e.g. its own width) and withmask 16 replaced with mask 17 (which might be mask 16 translated e.g. bya distance corresponding to the pixel spacing of the LCD 8 (FIG. 3)).

It should be understood that the mask 16 is made specifically for theLCD that will be used in the finished autostereoscopic display. The setof opaque mask areas 16 r correspond in position and spacing to pixelset 9 r; the set of clear areas 16 l correspond to pixel set 9 l. Mask17 (FIG. 7) is of near identical fabrication where the opaque areas 17 lcorrespond in position and spacing to pixel set 9 l; the set of clearmask areas 17 r correspond to pixel set 9 r. Masks 13 l and 13 r areused to define the respective viewing apertures 5 l and 5 r (FIG. 2)respectively.

A configuration for the two exposures is shown in FIGS. 6 and 7, whichare schematic perspective drawings and are not to scale. They representthe various elements as they would most likely appear on a an opticaltable —i.e. with everything turned on its side so the plane containingthe (horizontal) optical axis will be the same as that containing the(vertical) optical axis in actual use.

FIGS. 6 and 7 should be contrasted. After the first exposure shown inFIG.6 the mask 13 l is replaced with mask 13 r (FIG. 7) (or, which isequivalent, the mask 13 l will be translated downwards by its ownwidth). Similarly the mask 16 is replaced by mask 17 (or, which isprobably equivalent, the mask 13 l will be translated horizontally sothat the clear areas 17 r are in the same position as that previouslyoccupied by the opaque areas 16 r). A second exposure is then made(either on the same holographic plate 10 or on a second plate, in thefollowing we will continue to assume that just one double exposed Hlhologram is to be used). The hologram is then processed in theconventional way for the best signal to noise ratio.

The H1 hologram is then used to make an image planed H2 hologram 3 asillustrated in FIG. 8:

H1 hologram 10 is replayed with beam 20, which is the conjugate of thereference beam used in the recording. It reconstructs a real imagecomposed of:

Real image 24, which is the image of the diffuser 12 (FIGS. 5-7) asmasked by the mask 131 (FIG. 5).

Real image 25, which is the image of the diffuser 12 (FIGs.5-7) asmasked by the mask 13 r (FIG. 7).

Real image 22, which is the image of the mask 16 (FIG. 6).

Real image 23, which is the image of the mask 17 (FIG. 7).

It will be appreciated that the images 22 and 23 are images of opaque,therefore dark, objects. The effect of these images 22 and 23 is tolimit the positions whence the images 24 and 25 may be seen—as will beclear from considering the recording geometries described above.

In addition there is provided holographic material (say plate) 3comprising a substrate 27 and a suitable photographically sensitivelayer 21. There is also a reference beam 26 for the plate 3, which inthe illustrated case is diverging but need not be so. The sensitivelayer 21 would normally be applied to the face of the substrate 27placed closest to the real images 22 and 23. The optical distancebetween the holographic layer 21 and the images 22 and 23 should be thesame as the optical distance between the layer 21 and the plane of thepixels 9 of the LCD 8 (FIG. 3) in final use.

(Note on aberrations: in this configuration accurate reconstruction isimportant. All the conventional precautions should be taken e.g., thereconstructing beam 20 should be an exact conjugate of the referencebeam 11, the H1 hologram 10 should be flat, and the processing of thehologram 10 should not introduce dimensional change in the sensitivelayer. In addition aberrations caused by the thickness of the substrate27 to the H2 holographic plate 3 may have to be pre-corrected byintroducing a plate of similar optical properties to the H2 3 and in thesame relative position with respect to the H1 10 in the recording of theH1 10. Similar pre-correction techniques might be required to compensatefor refraction in the LCD substrate).

The holographic plate 3 is exposed and processed for good diffractionefficiency with minimum noise. Holographic plate 3 may then be used inan autostereoscopic display. When it is replayed with light source 1(where in the described case the angle of incidence of light 2 willilluminate it at roughly the same angle as the angle of incidence of thereference beam 26) it will reconstruct images 5 l and 5 r (FIG. 2) whichare themselves images of images 24 and 25 which in turn are images ofdiffuser 12 seen through apertures 13 l and 13 r (FIGS. 5-7). In thiscase the LCD 8 (FIG. 3) is disposed close to the hologram 3 such thatimages 22 and 23 (which are images of masks 16 and 17 (FIGS. 6 and 7)form within or very close to the pixel sets 9 l and 9 r (FIGS. 3 and 4)respectively and without light forming image 5 l (FIG. 2) passingthrough any member of pixel set 9 r (FIGS. 3 and 4) nor light formingimage 5 r (FIG. 2) passing through any member of pixel set 9 l (FIGS. 3and 4).

There is a choice of holographic materials that may be used in all theabove stages e.g. silver halide, dichromated gelatine, photopolymer,photo resist, thermoplastic, the optical arrangements are essentiallythe same for all suitable materials including, presumably, ones that areyet to become available.

The H2 3 can be used as a tool to replicate functionally identicalholograms. Optical methods can be used—in particular contact copying,alternatively mechanical methods such as embossing and moulding may alsobe used. In the mechanical cases the H2 3 would be made so that theinterference pattern was recorded in the form of a surface reliefstructure, which after a number of known intermediate stages, becomes amoulding or stamping tool. Moulding is particularly attractive if otheroptics are to be incorporated into one moulded unit—e.g. in the casewhere the H2 3 is made as a Substrate Guided Wave (SGW) hologram.

As has already been mentioned the described method of recording theHologram 3 is only one of many possible methods to achieve the desiredeffect. There exist one step methods (whereby the hologram 3 is madedirectly, without the need for the step involving H1 10). There alsoexist variations when the hologram 3 is to be placed between the LCD 8and the viewer (the configuration illustrated in FIG. 3) in which casean obvious variation is to place the holographic plate 3 on the otherside of the image 22 and 23 but with the sensitive layer still beingdisposed closest to the image 22 and 23. Once the basic principles havebeen understood a competent holographer would be able to design a rangeof methodological variations with relative ease. The aforesaidnotwithstanding we will now summarise some variations and some importantdesign considerations:

The hologram 3 can be produced by making a first master hologram withthe diffuser in a first position associated with the left viewing zone,and a second master hologram on a different holographic plate with thediffuser in a second position associated with the right viewing zone.The hologram 3 is then recorded as a transfer hologram from the twomaster holograms. The first master hologram is recorded as a transferhologram with a mask of alternating dark/opaque lines masking thetransfer hologram plate. The mask is then moved to uncover a differentset of lines on the transfer hologram plate and the second masterhologram is recorded onto the transfer hologram plate.

Holograms operate by diffraction and can produce pronounced chromaticdispersion. It is important to control this effect otherwise theperformance and/or usability of the display will be very severelycompromised. The use of a viewing zone formed as the diffuse imagedescribed allows the use of some simple methods to counteract theotherwise detrimental effects of chromatic dispersion. In e.g. FIG.8 itwill be noted that the plane of the diffuser aperture images 24 and 25is inclined with respect to the hologram 3. It is advantageous to usethe angle of inclination known to holographers as the achromatic anglewhereby the spectra of chromatically dispersed image points overlap eachother forming a largely achromatic image which is sharp in depth andfrom side to side.

There is a variation on this approach (FIG. 7a refers) where instead ofusing a homogeneous inclined diffuser 12 a similar effect may beproduced by using a plurality of diffuse lines of light 12 a, 12 b, 12c.etc. These are disposed so that each line lies in a plane inclined atthe achromatic angle, their length equals the width of the planediffuser(s) that would otherwise have been used. They are advantageouslydisposed parallel to the plane of the hologram and perpendicular to theplane of incidence of the illuminating light. When such an arrangementis illuminated by white light each line is chromatically dispersed intospectrum that is inclined at the achromatic angle, the respectivespectra of each of the diffuse lines overlap so as to produce an areawhere the color mix thereby produced renders a diffuse white(“achromatic”) zone functionally identical (under white lightillumination) to the zone produced by a homogeneous diffuser.

The lines 12 a. . . 12 n (FIG. 7a) will have to be diffuse so they canbe considered diffuse zones.

The most important general point is that the images of diffuse zoneswith appreciable width and height must be reconstructed at a distancefrom the hologram. The zones should be precisely located in distancefrom the hologram (i.e. minimal blurring along the z-axis). They shouldbe evenly illuminated (i.e. substantially the same brightness acrosstheir functional width) and no significant blurring in the horizontaldirection (i.e. the direction perpendicular to the plane of incidence ofthe illuminating light). The devices described herein allow all theseimportant performance criteria to be achieved. Failure to achieve any ofthem leads to restriction of the available stereoscopic viewing zone andthe introduction of highly undesirable artifacts such as color fringingand crosstalk between the left and right channels. A substantial regionof achromatic response up and down is usually required but it will beappreciated that the appearance of color distortion at the verticalextremes of the viewing zone is not a significant practical problem.

The use of the achromatic angle is one of several methods used to dealwith the effects of chromatic dispersion.

If the H1 10 and H2 3 are made with a diffuser aperture 13 arranged atan angle that is not close to the achromatic angle then other means ofdealing with chromatic dispersion must be used (unless, that is, thedisplay is to be use to display monochromatic images and the lightsource 1 has a narrow spectral bandwidth). Alternative methods include:

Full color holography using a thick holographic recording medium wherethe Bragg's angle condition of the hologram 3 effectively suppresses allbut the desired range of wavelengths. This is practical with bothreflection and transmission versions of the hologram 3.

Reconstruction of hologram 3 with a plurality of narrow spectralbandwidth light sources 1 r, 1 g, and 1 b where the sources may or maynot be disposed so that they each have different angles of incidencearranged so that the reconstructed images 5 r, 5 g, and 5 b overlap inspace with sufficient precision.

Use of dispersion compensation techniques in the replay of hologram 3,this involves the use of at least one other diffractive optical element(typically one made using holographic techniques) designed and disposedto introduce equal and opposite chromatic dispersion. This arrangementwill often require (a) that the aggregate optical power of the hologram3 and the additional optical element should be low and (b) the use of anadditional optical element (e.g. an lens or mirror) with positiveoptical power, in addition a louvered screen may also be requiredbetween hologram 3 and the additional diffractive optical element inorder to suppress the zero order which might otherwise be visible to theviewer.

Use of an additional holographic / diffractive optical element whichprovides several beams of light 2 r, 2 g, and 2 b focused differently sothat the respective red, green, and blue images of the viewing apertures5 (FIG. 5) overlap with sufficient precision.

An interesting variation on a previous configuration is to complicatethe recording process as follows: Three diffuse lines are used(corresponding to 12 a, 12 b, 12 c in FIG. 7a), these should be disposedso that from a given viewing position their respective dispersed imagesare seen as red, green and blue —the aggregate being white. As well asbeing divided between pixel sets 9 l and 9 r they are also dividedbetween pixel subsets 9 l red, 9 l green, and 9 l blue and 9 r red, 9 rgreen, and 9 r blue in such a way that the pixels of LCD 8 will bedivided providing red, green or blue images to the left eye andsimilarly red, green or blue images to the right eye. When used inconjunction with a grey scale LCD addressed much as if it were a colorLCD, then the displayed image will also be in full color. The colorrendition will be very similar to that displayed by “full color”“rainbow” holograms such as the popular embossed holograms in currentuse. If a reflection hologram is used the color rendition will be morestable in the vertical direction. This method is certainly complicatedby the fact that 6 masks (16 red, 16 green, 16 blue, 17 red, 17 greenand 17 blue) are required but is nevertheless perfectly feasible.

In the preceding descriptions it has been assumed that when the hologram3 is illuminated by a single light source then one stereoscopic viewingzone will be created. It is possible to make the hologram 3 in such away as to provide a plurality of such positions without recourse tomultiple light sources. This requires the recording of a plurality ofdiffuse zones for each left and right view. In all other respects theholographic methods are the same. FIG. 15 illustrates the end resultwhere light source 1 illuminates the LCD and hologram combination 3 and8. In this case the hologram 3 has been made with diffusers in threepositions for the left viewing zone and in corresponding three positionsfor the right viewing zone. In use the images of these diffusersreconstruct to form three separate stereo viewing zones where the leftand right pair 51 a and 5 ra form in one position thereby providing onestereo viewing position while the other pairs 51 b , 5 rb and 51 c, 5 rcsimultaneously form two further stereo viewing positions. It will beobvious that the distance and height of each of stereo viewing positionsfrom the hologram 3 need not be identical and that any number of suchstereo viewing positions can be provided.

There are a number of ways of making the H2 3, which are worthy ofdiscussion here.

The case described above and drawn in FIG. 8 uses the well know off axistransmission hologram technique where the reference beam 26 andreconstructing beam 2 illuminate the hologram 3 through free space(notwithstanding the introduction of mirrors or lenses to assist in theoptical performance and/or practical construction of the display). Analternative is to use a substrate guided wave (SGW) technique where thelight beam is guided by the hologram substrate 27, frequently by usingthe phenomenon of total internal reflection.

Two basic arrangements are illustrated in FIGS. 9 and 10 (these drawingsare not to scale). In comparing these to FIG. 8 it will be noticed thatthe reference beam (here represented by just one ray 26) enters thesubstrate 27 of hologram 3 by way of a bevelled edge. (It will beunderstood that there are a number of other ways of coupling the beam 26into the substrate including prisms and diffraction gratings.) The beamthen suffers internal reflection and impinges on the sensitive layer 21where it interferes with the object light to create a transmission typehologram. FIG. 10 illustrates the same configuration but with thesensitive layer 21 located on the other side of the substrate 27, suchan arrangement produces a reflection hologram which, for full color use,should be operate in three appropriately chosen wavelengths.

As well as producing a more compact display the SGW mode can be used tointroduce other optical elements, which if the holographic surface in amechanically reproduced version is in the form of a surface relief,allow for very economical manufacture and complex optical performance.We will highlight a few possibilities.

FIG. 11 is a perspective view of a SGW transmission hologram 3 (but can,mutatis mutandis, be in reflection mode). The light source 1 is mountedin or adjacent to the hologram 3, which comprises the holographicsurface 21 and a substrate 27 with further optical properties. The light2 emitted by the source 1 passes down through the substrate 27 until itstrikes the bottom surface 28 which is curved and equipped with areflective coating. The beam 2 is then reflected, focused and directedtowards the holographic surface 21 where it is diffracted formingwavefronts 4 which forms images 5 l and 5 r (FIG. 2) as previouslydescribed. It will be appreciated that we have only illustrated certainrays of light for the purpose of clear illustration. The configurationshown in FIG. 11 has a number of practical advantages. It can be made asone unit by moulding techniques where the holographic surface 21 can becreated as a relief structure in the moulding process. The light beam 2can be focused using the moulded structure, both by the mirrored surface28 and (which is not illustrated) at the point of insertion into thesubstrate 27. A plurality of light sources can be used to enable theviewing zones 5 to be moved providing the ability to track a movingviewer and allowing full resolution 2D operation.

As an alternative to using multiple sources for 2D compatibility aconventional diffuser backlight can be provided behind the hologram 3which will be clearly visible through the holographic surface 21 and thesubstrate 27. Such a system can be economic in manufacture as well asbeing robust and compact.

FIG. 12 illustrates another variation, which can be conveniently (thoughnot necessarily) made as a SGW hologram. The light 2 m emitted by source1 first passes through an optional focusing optic (e.g. a lens), entersthe substrate 27 and impinges upon an optic 29 on the far face of thesubstrate 27. The optic 29 performs at least the function of directingthe light 2 n such that the redirected light 2 p impinges upon theholographic surface 2 l at the correct angle. In addition to this basicfunction the optic 29 can be made as a Holographic Optical Element (HOE)which effects dispersion compensation when combined with hologram 21. Inthe case illustrated the optic 29 can be a surface relief transmissionholographic optical element, which has a reflective coating on its outersurface. Optic 28 can be used to assist the dispersion compensation byproviding optical power if the hologram 21 in combination with the HOE29 has no optical power. (Alternatively the element 29 can be areflection HOE or indeed a reflective optic, which redirects and,optionally, focuses the light 2 n.)

There are many further variations on the use of substrate guidedillumination of the hologram 3.

There will also be cases where equivalent optical performance can beachieved by computing the fringe structure of hologram 3 and fabricatingit using non-optical procedures (e.g. e-beam).

As mentioned earlier, the holographic optical element 3 may befabricated using a “one step” technique. One step methods maybeimplemented in a number of ways. A simple one is illustrated in theaccompanying FIG. 13. Instead of first making a master (H1) hologramfrom which image planed H2 copies are made (i.e. the copies arepositioned within the image reconstructed by the H1) we make a hologram,which can itself be used with the LCD. In the illustrated case thehologram is masked by the masks 16 and 17 in two exposures where themask is in very close contact with the sensitive layer and where theposition of the diffuser mask 13 is moved between exposures as describedpreviously.

There is a another class of one step holograms which use large focusingoptics—e.g. large aperture low f/number lenses—to focus a real image ofthe object to be recorded, the holographic plate can then be put“inside” the real image.

Any 3D image provides objective information regarding the size of theobject represented. In an autostereoscopic display the size of the imageis restricted by the perceived size of the display. It is advantageoustherefore to enlarge the apparent size of the display while retainingthe autostereoscopic capability. A method of doing this with the presentbasic display will be described, it can be imagined as a window thatreveals a three dimensional scene beyond it, just as a large object canbe seen through a tiny window, so the 3D image of the object can be seenfull size through the relatively small autostereoscopic “window” to bedescribed.

Referring to FIG. 14, line 30 represents the autostereoscopic displayalready described incorporating light sources 1, LCD 8 and hologram 3.The plane of the LCD 8 and Hologram 3 is indicated. The hologram can bemade to provide a real image of the zone 5 l and 5 r (FIG. 2) or, as isthe case in FIG. 13, they can be virtual images 31. An optical element32 with positive optical power is provided and positioned so that thescreen 3 or 8 of the display 30 lies between the optical element 32 andits focal plane. The simple ray tracing indicated by dotted lines showsthat an enlarged virtual image 33 of the screen 3 or 8 is formed. Thisimage 33 will be advantageously positioned such that it lies at asimilar distance from the viewer 35 as the three-dimensional imagegenerated by the stereoscopic principles used (thereby minimizing anyconflict between stereopsis and accommodation). The virtual images 31are focused by the optic 32 to form real images 34, convenientlypositioned at the intended viewing position. According to the precisegeometry used the location of the virtual images 31 can be adjusted and,in some cases, may be found to be advantageously formed first as realimages, which remain real images when refocused by optic 32. The images31 can also be located at infinity.

The functioning of such a display can then be understood to be the sameas previously described where each eye will see a different picture and,provided the pictures are a stereo pair, the viewer will see a threedimensional image beyond the optic 32 as if beyond a window. The optic32 can be refractive, reflective or diffractive (In each case the FIG.14 would have to be altered in the appropriate way, though the imagewould form in functionally the same way.)

It is advantageous if the optical properties of the element are arrangedso as to blur the pixels of the LCD. This can be achieved by introducingminimal aberrations such as those that occur very readily with a Fresnellens. In the case of a diffractive element this can be achieved bydesigning such that a slightly blurred image is produced or by using theblurring that occurs though chromatic dispersion.

The use of a diffractive optical element (which would presumably be aholographic optical element (HOE) in this case) has certain advantages:The HOE can be transparent thereby enhancing the impression of a windowand allowing the image to be superimposed on a real scene viewed beyondthe window. HOEs can also be made with multiple independent focal pointscreating another way of allowing more than one viewer to enjoy the scenesimultaneously. HOEs usually operate off axis, which allows forconvenient and elegant construction. They also allow controllable waysof de-pixelating the image (by enlarging the source of one or more ofthe beams used to make the HOE.)

If a HOE is used then a number of problems need to be addressed. Inparticular, assuming the display is called upon to produce full colorimages then the problem chromatic dispersion must be addressed. Onesolution is to use a HOE tuned to three appropriate wavelengths. Anotheris to use monochromatic light in three wavelengths provided byappropriately chosen light sources or by filtration. According to theway the HOE is made this may mean that three grey scale autostereoscopicdisplays 30 would have to be used and may also mean that they need to belocated in different positions optically (which can be achievedmechanically, optically with wavelength selective mirrors or opticallywith additional diffractive elements).

The display device may also comprise a system for the independent eyetracking of one or more viewers (as described more fully in ourapplication PCT/GB93/01709). As the viewer(s) move their heads, thelight source is (effectively) moved in order to position the viewingzones correctly.

What is claimed:
 1. A display device comprising: a holographic opticalelement having at least two sets of hologram regions, the regions of thefirst set being interleaved or overlapping with adjacent regions of thesecond set and being constructed such that light incident on each set ofregions is diffracted so as to construct a respective one or more of aplurality of real or virtual images of a diffuse light source; and imagegenerating means including a plurality of image elements; wherein theholographic optical element and its hologram regions are disposed anddesigned such that light having passed through a first set of imageelements of the image generating means is diffracted by the first set ofhologram regions, and light having passed through a second set of imageelements of the image generating means is diffracted by the second setof hologram regions.
 2. The display device according to claim 1,wherein: the image generating means includes a liquid crystal array andmeans to illuminate the liquid crystal array, and each image elementincludes one or more pixels of the liquid crystal array.
 3. The displaydevice according to claim 1, wherein the holographic optical element isspaced from the image generating means.
 4. The display device accordingto claim 1, wherein the holographic optical element lies adjacent theimage generating means.
 5. The display device according to claim 1,further comprising: means to vary the illumination of the imagegenerating means, whereby viewing zones formed by the real or virtualimages are moved.
 6. The display device according to claim 1 wherein:the image elements define a left hand and a right hand stereoscopicimage, each of which is directed to a respective viewing zone; and thedisplay device provides a stereoscopic image in use.
 7. The displaydevice according to claim 1 further comprising: a focusing optic whichcreates a virtual image of the plurality of image elements and theholographic optical element and forms real images of viewing zones, eachimage being viewed in use through a respective one of the real images ofthe viewing zones.
 8. The display device according to claim 1, whereinthe plurality of images together form a two-dimensional image.
 9. Thedisplay device according to claim 1 further comprising: a plurality oflight sources which illuminate the holographic optical elementsimultaneously whereby a plurality of viewing positions are provided.10. The display device according to claim 1, wherein the imagegenerating means comprises a monochrome image generating means wherebycolor images are displayed on the image generating means.
 11. Thedisplay device according to claim 1, wherein the image generating meansis combined with the holographic optical element arranged such thatcertain regions of the holographic optical element are associated justwith the image elements displaying one color separation and otherregions of the holographic optical element are associated just with theimage elements displaying other color separations, whereby thewavelengths of light diffracted by each set of regions towards a viewercorrespond to the color of the color separation displayed by the imageelements associated with the color separation, the whole being arrangedso as to display a full color two dimensional or three dimensionalimage.
 12. The display device according to claim 1, wherein each set ofregions comprises: an array of regions.
 13. The display device accordingto claim 1, wherein each set of hologram regions comprises: aholographic recording of one or more diffuse light sources in one ormore of a plurality of different positions with respect to theholographic optical element.
 14. The display device according to claim1, wherein the holographical optical element is constructed as asubstrate guided wave device.
 15. The display device according to claim1 wherein each set of reigions comprises: a holographic recording whichhas been made using a mask having an array of apertures.
 16. The displaydevice according to claim 1, wherein each set of regions diffracts lightincident on the holographic optical element to form an image of an arrayof apertures.
 17. The display device according to claim 1, wherein theholographic optical element is a surface relief hologram made by moldingor embossing.
 18. The display device according to claim 1, wherein theholographic optical element is a volume hologram made in photopolymer,dichromated gelatine or silver halide.